1. Field of the Invention
This invention relates to heat and flame resistant, flexible, insulating fabric compositions. More particularly, it relates to refractory coated porous base fabric compositions and their method of preparation. This invention especially relates to refractory coated porous base fabric compositions wherein the refractory coating is formed on the fabric by plasma spraying.
2. Description of the Prior Art
Asbestos and other thermal protection materials are employed as protective mats for welding operations, furnace linings, fire-resistant linings and the like to provide protection from flames and themal extremes as well as for dissipation of heat at a high rate. Although in each application reasonable performance is obtained, the selected material has limitations in its range of uses and in its effectiveness, even in uses for which it is particularly adapted. In addition, asbestos has found less utility in certain uses because of recent findings that it may constitute a health hazard.
Ceramics and/or glass fibers have been used heretofore to prepare high temperature electrical insulating tape. In addition, coated fabrics and ceramic structures have been prepared with a combination of glass fiber fabrics and metal oxides.
U.S. Pat. No. 4,282,284 and commonly assigned patent applications Ser. Nos. 225,888 and 294,770 disclose a heat resistant, flexible, refractory, insulating fabric composition of a porous base fabric, preferably a knitted fiberglass fabric, coated with heat resistant refractory materials, such as alumina and zirconia, and an organic bonding agent, such as acrylic latex alone or admixed with colloidal silica. A polymeric coating, such as polyvinyl chloride, may be formed over the coated fabric to provide abrasion resistance to the composition. A feature of this composition resides in the fact that the refractory materials are bonded in the interstices as well as to the surface of the base fabric, such that a significant amount of the fabric's flexibility and stretch properties are retained. This fabric composition has a variety of uses and therefore may be employed as the dielectric in insulated electrical wire or cable, as protective mats and curtains in welding operations, as linings for fire resistant machine and appliance covers, as duct and pipe insulation, as wrappings for engine exhaust systems and the like. When this fabric composition is exposed to heat and high temperatures, the organic bonding agent containing the refractory materials will decompose, causing the refractory materials to fuse into the softened surface of the knit fiberglass base fabric, enabling it to withstand intense heat and elevated temperatures well beyond the normal melt temperature of the fiberglass fabric. The resulting fabric structure will have ceramic qualities and will not soften, melt, drip or lose its insulating properties.
Although the organic components of the bonding agent of these prior art fabric compositions may not produce a flame when exposed to a flame or the intense heat developed by a flame, i.e. temperature of 1750.degree. F. and above, they will decompose at these temperatures resulting in some products of combustion in the form of smoke and fumes which may prove harmful to persons in the vicinity.
Commonly assigned patent application Ser. No. 294,771 discloses improved refractory coated fabric compositions which eliminate the products of combustion which may be formed with the fabric compositions described above. These improved fabric compositions are similar to those which produce products of combustion in all respects except that an inorganic bonding agent is substituted for the organic bonding agent of the prior art fabric compositions. When these fabric compositions with the inorganic bonding agent are exposed to high temperatures above about 1750.degree. F., no products of combustion in the form of smoke or fumes are produced.
The refractory coating formed on the prior art compositions of the above commonly assigned patent and patent application is discontinuous so as to provide flexibility to the finished composition. A knitted fiberglass is the preferred porous base fabric of these compositions since the refractory coating impregnates the interstices and coats the outer surface to form a discontinuous layer. A woven fiberglass fabric is not as desirable since a discontinuous layer is more difficult to form on a fabric having a woven configuration.
Although ceramic coated porous fabric compositions prepared with knitted fiberglass are preferred, the thickness of these compositions limits their usefulness. Even when employing a 31/2 oz./yd.sup.2 fiberglass fabric which is made from a D-450-1/0fiberglass yarn, refractory coated compositions prepared by such prior art methods as reverse roll coating are 10-11 mils thick and weigh 71/2-8 oz./yd..sup.2 A thinner composition would find increased utility in the electrical and aerospace industry where weight is an economic disincentive and space limitations are often a controlling factor. For example, a thinner refractory coated composition would permit the preparation of thinner and lighter insulated electrical conductors.
Plasma spraying is a known coating deposition technique. Coatings of metals, metal alloys, refractory metals and ceramics may be applied to substrates by plasma spraying to provide such desirable properties as wear resistance and corrosion resistance. In plasma spraying a regulated electric arc, developed within a plasma spray gun, excites a stream of inert gas, such as argon or nitrogen, producing within the spray gun a thermal plasma spray at a controllable temperature of 12,000.degree.-30,000.degree. F. Powdered materials are introduced into the plasma where they are heated to a plastic or molten state, then ejected from an orifice on the front of the gun and are impringed at a very high velocity onto a properly prepared substrate to form a bonded coating. U.S. Pat. Nos. 3,016,477 of Gage et al. and 4,173,685 of Weatherly disclose the plasma spraying of metals and refractory metals onto metallic substrates. U.S. Pat. Nos. 4,121,082 of Harrington et al. and 4,121,083 of Smyth disclose that metals, metal alloys, ceramics, such as metal oxides, and carbides may be plasma sprayed onto a substrate.
Although U.S. Pat. No. 3,669,719 of Doede et al. teaches plasma spraying onto non-metallic and metallic substrates, including such non-rigid materials as glass fiber reinforced plastics and rubber also reinforced with glass fibers, the material being plasma sprayed is always a metal. The plasma sprayed metal coatings of Doede et al. are from about 0.002 to about 0.050; preferably about 0.005 to about 0.025 inches in thickness. U.S. Pat. No. 2,714,563 of Poorman et al. is not related to plasma spraying but discloses the coating of metals, alloys or ceramic materials onto such non-rigid materials as paper, plastic or cloth by detonation gun spraying. Specific examples include coatings of aluminum and zinc onto cotton cloth. Poorman et al. does not suggest that plasma spraying can be used to prepare the same products. U.S. Pat. No. 4,173,685 of Weatherly discloses that a particular coating of nickel base alloys and metal carbides can be applied to a metal substrate by either plasma spraying or detonation gun spraying.
None of these prior art patents teach the application of ceramic materials to a non-rigid substrate such as fiberglass by the coating deposition technique known as plasma spraying.
It is an object of this invention to provide refractory coated fabric compositions having a thickness substantially thinner than those known in the art.
It is another object of this invention to provide a process for preparing refractory coated fabric compositions wherein the refractory coating is substantially thinner than similar compositions prepared by roll coating procedures.
It is a further object of this invention to provide a process for forming 1 to 2 mil thick refractory coatings on porous fiberglass fabrics.